This invention relates to an electronic converter drive for a switched reluctance motor.
Switched reluctance motors conventionally have poles or teeth on both the stator and rotor (i.e. doubly salient). There are phase windings on the stator but no windings on the rotor. Each pair of diametrically opposite stator poles is connected in series to form one phase of the switched reluctance motor.
Torque is produced by switching current on in each phase winding in a predetermined sequence that is synchronized with the angular position of the rotor, so that a magnetic force of attraction results between the rotor and stator poles that are approaching each other. The current is switched off in each phase before the rotor poles nearest the stator poles of that phase rotate past the aligned position, otherwise the magnetic force of attraction will produce a negative or braking torque.
The torque developed is independent of the direction of current flow so that unidirectional current pulses synchronized with rotor movement can be obtained from an converter using a unidirectional current switching element such as a thyristor or transistor.
Each time a phase of the switched reluctance motor is switched on by closing a switch in a converter, current flows in the stator winding, providing energy from a dc supply to the motor. The energy drawn from the supply is converted partly into mechanical energy by causing the rotor to rotate towards a minimum reluctance configuration and partly into stored energy associated with the magnetic field. After the switch is opened, part of the stored magnetic energy is converted to mechanical output and part of the energy is returned to the dc source.
Net energy delivered by the dc supply (total energy supplied to the machine less the energy returned to the dc supply) in one cycle may be illustrated graphically and is equal to the area enclosed by the trajectory of flux-linkage versus current. This net energy is closely related to the mechanical output energy of the motor. It is desirable to maximize the enclosed area of the trajectory and therefore maximize mechanical output energy for a given peak current, not only to maximize utilization of the peak current capability of the converter switching device but also increase efficiency since high peak current causes saturation and increased iron losses.
In the simple commutation process of switching on one phase of the converter and building up flux and then switching off the phase and having flux decay there is very little control over the shape of the flux-linkage/current trajectory. The only control variables are the times when the converter switching device is switched on and off, and the magnitude of the dc supply voltage. Once these parameters have been chosen, the flux-linkage/current trajectory for any given motor speed will be fixed, and in general will have an area much smaller than the ideal theoretical maximum.
One approach which has been proposed in the past to limit the peak current is to limit the dc supply voltage by switching the converter transistor off when the current reaches a preset level and switching it on again when the current falls below a somewhat lower level. This can be done with no additional switching elements and only requires the addition of a current-sensing transducer and logic control circuitry. This strategy has been used not so much for increasing the area of the flux-linkage/current trajectory, as for limiting the peak current at low speeds, when there is insufficient back-emf from the motor to accomplish this limitation. The effect of "chopping" on the flux-linkage/current trajectory is to improve its shape because it occupies a larger fraction of the theoretically available area. However, beyond the maximum current at maximum flux linkage, the trajectory is not under the control of the chopping controller and is no better than that of the simple commutation process.